1. Field of the Invention
The present invention relates to an apparatus for determining a wall (or shell) thickness of a microcapsule nondestructively and conveniently by light-scattering techniques, and a method for the same.
2. Description of the Related Art
As a method for determining (or measuring) a wall thickness of a microcapsule, there have been known a method which comprises enveloping and embedding or solidifying a microcapsule with an epoxy resin, paste, or the like, then breaking the microcapsule, and observing the broken-out section of the solidified matter by a scanning electron microscope (SEM) to measure the wall thickness with the eye, and a method which comprises cutting a microcapsule by a microtome to give an ultrathin section, and observing the cross section of the ultrathin section by a transmission electron microscope (TEM) or an optical microscope (OM) to measure the wall thickness with the eye.
However, these methods have some problems as follows: (i) since the microcapsule is very small in size, it is difficult to break the enveloped and embedded microcapsule certainly; (ii) in cutting, if the capsule wall is hard, the wall is often broken or crushed, and if the capsule wall is soft, the wall is often deformed; (iii) it is necessary to dry the microcapsule for breaking or cutting since the microcapsule is usually formed in an aqueous phase or an oil phase, however, the capsule wall is deformed by dryness; (iv) even when the microcapsule is breakable, the capsule involving a liquid therein has possibilities that the liquid vaporization adversely affects the electron gun in SEM or TEM observation; and (v) since the thickness observed by the microscope is only based on a local field of view, it is difficult to obtain a statistical information on the basis of a wide field of view. These methods therefore not only need accurate and complicated operations such as breaking or cutting but also find difficulty obtaining a reliable data adapted to meet the actual situation. Furthermore, it is extremely difficult to measure a thickness for a polydisperse microcapsule statistically.
Meanwhile, it is known to analyze a structure of a microcapsule by light scattering techniques. In the light scattering techniques, a microcapsule can be analyzed without destruction. In the ordinary light scattering techniques, the outer diameter of the microcapsule can be determined, but the capsule wall thickness cannot be determined.
Dobashi et al. derived a capsule wall thickness by irradiating a single microcapsule with a light, and conducting a fitting analysis to a dependency of a scattering angle on the resultant scattered light intensity on the basis of Mie Scattering Theory (Dobashi et al., Langmuir Vol. 14, pp. 745 to 749 (1998)). However, since this method utilizes a complicated theoretical equation (scattering function) in which the inner diameter and the outer diameter are the variables, complicated operations are required and the fitting analysis sometimes becomes difficult depending on the wall thickness of the capsule, or others. Moreover, in this method, it is extremely complicated to derive the wall thickness of a large number of microcapsules having a particle size distribution even if it is possible, and it is difficult to calculate the thickness adapted to meet the actual situation.